Additional features of the vascular structural changes associated with the development of pulmonary hypertension in the rat will be followed - mitotic activity of the endothelial cell and of the muscle cell and its precursors. The 'recovery' pattern from hypoxia will be followed and compared with that after Crotalaria spectabilis feeding. From indwelling catheters in the pulmonary artery the pattern of regression in the pulmonary hypertension induced by hypoxia will be studied and the development of hypertension in tha young animal compared with the adult. The behavior of the restricted circulation on exercise will be followed. The effect of an aorto-pulmonary shunt in the young piglet leads to impairment of growth of the pulmonary circulation and to its increased muscularity. The structural response to correction of the shunt will be analysed. Detailed analysis of structural changes is being made in four disease groups - SIDS, respiratory distress of the newborn, certain congenital cardiac defects and shock lung of the adult.